An on-demand type ink-jet print head is disclosed in the U.S. Pat. No. 3,946,398 entitled "Method and Apparatus for Recording with Writing Fluids and Drop Projection Means Therefore", issued to Kyser et al. There, an ink droplet is formed by applying a driving pulse to a piezoelectric element installed on a pressure chamber of the ink-jet print heat. As disclosed in the U.S. Pat. No. 4,281,333 entitled "Ink-On-Demand Type Ink-Jet Printer with Coordinated Variable Size Drops with Variable Charges", issued to the present inventors, the droplet size (volume) may be varied by controlling the energy content of a drive pulse. For example, the amplitude and/or the width of the driving pulse may be varied to change the size of a dot recorded on a recording medium.
Therefore, in an ink-jet print head, a half-tone recording can be provided by controlling the energy content of a driving pulse, in response to a signal representing the density of an image to be recorded. However, a droplet-volume range is not so wide that the volume of the ink droplet can be adaquately varied. For example, the volumetric ratio of the maximum-sized ink droplet to the minimum-sized ink droplet is between 2-3, at the greatest. Accordingly, the half-tone reproduction range, in which an image recorded at a predetermined density can be reproduced by only varying the volume of the ink droplets, is restricted to about 0.4-1.5 in terms of reflection density. In particular, the half-tone reproducibility of the bright portions of an image is low.
Other known half-tone recording systems include the dither method and the density pattern method. In these methods, the number of dots is increased or decreased in accordance with the density of the image to be recorded, to provide a half-tone reproduction thereof. In order to increase the number of graduations in such an imitation half-tone reproduction system, the number of dots is increased per unit area of the image being subjected to the half-tone reproduction. However, when the dot-recording density of a recording apparatus is constant, the resolution drops.